FIG. 12 is a side view diagram by way of example, of a conventional process tube 1 for use in vertical furnaces.
The process tube 1 for vertical furnaces is of a quartz-made cylinder with its upper end closed and its lower end left open. A process gas introducing pipe 2 is connected to the center of an upper end portion of the process tube 1 for communication with an inner cavity of the process tube, and an exhaust pipe 3 is connected to a peripheral wall of the process tube 1 at a position near its lower end for communication with the tube's inner cavity through an exhaust port la formed at the same position.
A cap 4 is inserted into the lower open end of the process tube 1 for vertical furnaces while leaving an appropriate gap between itself and an inner circumferential surface of the process tube 1 for vertical furnaces. A wafer mount boat 5 is supported to an upper end surface 4a of the cap 4 directed toward the inner cavity of the process tube 1.
In other words, the cap 4 is supported at its lower end by a lifting device (not shown) and inserted into the process tube 1 for vertical furnaces from below, with the wafer mount boat 5 kept supported on the upper end surface 4a of the cap 4.
The cap 4 is inserted into the process tube 1 such that the upper end surface 4a is positioned above the exhaust port 1a. Thus, during the process carried out in the vertical furnace, the exhaust port 1a of the process tube 1 faces an outer circumferential surface 4b of the cap 4.
Accordingly, gas supplied to the process tube 1 via the process gas introducing pipe 2 flows downwardly and reaches the exhaust port 1a after passing the gap between the inner circumferential surface of the process tube 1 and the outer circumferential surface 4b of the cap 4, following which the gas is discharged to the exterior via the exhaust pipe 3.
In the above conventional process tube 1 for vertical furnaces, however, since the cap 4 is simply cylindrical in shape and the exhaust port 1a is defined at only one location in the circumferential direction, there occurs in the vicinity of the upper end surface 4a of the cap 4 a difference in exhaust rate between a region A near the exhaust port 1a and other regions remote therefrom. As a result, the temperature in the region A where the exhaust rate increases is more likely to be lower than in the other regions.
This has raised a problem that in-plane variations and wafer to wafer variations in thickness and quality of thin films formed near the cap 4 inside the vertical furnace, and of wafers annealed at that location may become significant.
To overcome the above problem, prior systems have attempted to carry out the process while rotating the cap 4 within the process tube 1 for vertical furnaces. However, this method is accompanied by a high risk that the inner cavity of the process tube 1 may become contaminated by dust or dirt which is produced due to the presence of sliding parts in the process tube 1.
Another approach which has been used is the connection of a plurality of exhaust pipes 3 to the process tube 1 for vertical furnaces so that the exhaust flow will not be localized. However, this method raises the problem that the layout of the apparatus around the process tube 1 is complicated or because of the complication layout, the layout must be changed.